Background: The treatment of preterm labor with drugs to arrest labor ("tocolysis") has become the standard of care in the United States. Magnesium sulfate (MgSO4) is among the most commonly used drugs for this indication. While MgSO4 is felt to have a tolerable side-effect profile, there is a risk of pulmonary edema (the most dangerous adverse event) with its use with estimates as high as 8%. This complication may lead to maternal adult respiratory distress syndrome or death, and can have secondary adverse fetal consequences as well. Specific Aims: 1) To examine in a retrospective cohort study the true incidence of pulmonary edema when MgSO4 is administered as a tocolytic. 2) To examine in a case-control study nested within the cohort the relationship between patient parameters and other clinical factors and the incidence of pulmonary edema in patients treated with MgSO4. 3) To utilize the results of the case-control study to develop and validate a predictive index that will define a high-risk group of patients for pulmonary edema associated with MgSO4. Methods: A retrospective cohort study will be performed to determine precisely the incidence of pulmonary edema in patients treated with MgSO4 for preterm labor. These patients will be identified by ICD-9-CM codes for preterm labor and delivery. The medical record charts of both cases and a random sample of non-cases will then undergo detailed review and data from these groups will be compared. The primary analysis of these case-control data will include unadjusted analysis and multivariable explanatory models, to provide insight into the etiology of pulmonary edema associated with MgSO4. Then, the data from the case-control study will be utilized to develop a clinical predictive index, in order to identify patients at high risk of developing pulmonary edema when treated with MgSO4. The predictive rule will then be validated in a separate patient population. The information from the predictive models will be useful to physicians to select appropriate patients for this tocolytic agent, i.e., those at low risk for pulmonary edema, and to guide their clinical management of patients receiving MgSO4.